Centronuclear myopathy (CNM)

Centronuclear myopathy (CNM) is a muscle disorder that is inherited in Labrador retrievers. It has been recognized in the retriever community for decades and known as "muscular myopathy." It is inherited as an autosomal recessive trait, meaning that the disease allele must be inherited from both parents in order to produce an affected puppy.

Carriers of CNM appear normal, but they will pass the mutant disease allele on to half of their puppies. Whenever two carriers are bred, there is a likelihood that affected puppies will be born.

At birth, affected puppies are indistinguishable from their normal littermates, but muscle weakness and muscle atrophy become evident at an early age. Some affected pups show disabling signs at six to eight weeks of age, while in others severe signs do not become evident until three to five months of age.

Affected puppies exhibit muscular weakness, an awkward gait and decreased exercise tolerance. Severely affected pups may have a low head carriage, and a gait characterized by short, stilted strides. The hind legs are often advanced simultaneously in a synchronous, bunny hopping fashion. Most affected puppies are euthanized due to poor quality of life.

Prior to 2003, the diagnosis of CNM required muscle biopsy of affected puppies. In 2003, Drs. Stephanie Blot and Laurent Tiret at the Alfort School of Veterinary Medicine in France identified the genetic mutation causing CNM and made a test available to Labrador retriever owners worldwide.

DNA testing can confirm that an affected puppy with symptoms has CNM. More importantly, testing can be used to determine whether healthy individual dogs are carriers of CNM (possessing one disease allele and one normal allele) or whether they are clear of the mutation. This is important information to have about a dog prior to breeding so that a breeder can avoid producing puppies affected by CNM.

• If a carrier is bred to a non-carrier, none of their pups will be affected, but 50 per cent will be carriers.
• If a carrier is bred to a carrier, approximately 25 per cent of the puppies will be clear of the mutation, 50 per cent will be carriers and 25 per cent will be affected and likely die of CNM.\

Testing for CNM: the process

Testing for CNM involves the collection and submission to the laboratory in France of an oral swab for DNA. This should be done by a veterinarian. The veterinarian will also verify the identity of the dog being tested (microchip or tattoo) for the laboratory.

Dogs that are clear of the CNM mutation will be published on the CNM website's list of clear dogs (no charge), and all results are eligible for posting on the Orthopedic Foundation for Animals (OFA) for a nominal charge.

Owners who would like to have their dog tested for CNM at the WCVM should do the following:

• Make an appointment with the VMC's Small Animal Medicine service for DNA testing. 

• Go to the CNM website at least four days before your appointment. Follow the prompts (Testing Your Labrador), answer the questions and pay the fee ($55 USD with a credit card).

• The laboratory will send you an email within 48 hours with a submission number and some paper forms. Print these forms and bring them with you to your appointment.

• The laboratory will also send you a package in the mail with some paper forms, mailing envelope and DNA brushes. This will probably not arrive before your appointment — we will use some that we have here at the WCVM Veterinary Medical Centre. We do ask, however, that you send the brushes to us when you get them to replace our supply. Send brushes to: 
  
  
  • Veterinary Medical Centre Pharmacy
    52 Campus Drive, University of Saskatchewan
    Saskatoon, SK S7N 5B4

• Do not give your dog anything to eat or drink (except water) for four hours before your appointment.

• During the appointment:
  • A veterinarian will collect a history and examine your dog. If your dog is due for routine procedures such as annual examination or vaccination these can be completed during the same visit.
  • Your dog's permanent identity will be verified (microchip or tattoo) if possible. If your dog has a tattoo please bring a copy of their CKC or AKC registration papers stating the tattoo number for your dog in case one of the letters/numbers is difficult to read.
  • We will perform the DNA swabs, fill out the required forms and seal the samples in envelopes according to directions from the laboratory.
  • We will give you the samples in a sealed, pre-addressed envelope to mail to the laboratory. We recommend mailing the CNM test via First Class Mail (Canada Post) to the French laboratory.

• The VTH fee will be the fee for an office call plus a $35.00 DNA collection and processing fee (includes the swabs). If more than one DNA test is done during one visit, (CNM plus EIC for example) there will not be an additional charge.